Greasemonkey was the good one and then something happened with it and it was no longer the best choice. I'm not sure what. The same repeated with Tampermonkey. It now has telemetry built into it, at one time w/ no privacy policy. https://www.reddit.com/r/firefox/comments/6hs59w/tampermonke... The good one now is Violentmonkey. Has been for some time. Does Mozilla have an allowlist for these? If so I don't like it.
A big thing for me is that Tampermonkey is not open source. And yes, Firefox on Android has an allowlist of "known working good" extensions, the reason given being that Firefox for Android doesn't implement everything the Desktop version implements and IIUC, want to a avoid users blaming the browser when something breaks because of it.
How does that even work? I assumed that Firefox extensions were implemented in frontend web technologies (JavaScript, CSS, etc.)? Do they use WebAssembly or something, or is it just obfuscated to the point that trying to read it isn't worthwhile?